The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 419.2 Mya (million years ago), to the beginning of the Carboniferous Period, about 358.9. It is named after Devon, England, where rocks from this period were first studied. The Devonian period experienced the first significant adaptive radiation of terrestrial life. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish reached substantial diversity during this time, leading the Devonian to often be dubbed the “Age of Fish”. The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment.
The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolving into legs. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida.
Climate and geography
The supercontinent Gondwana occupied most of the Southern Hemisphere, although it began significant northerly drift during the Devonian Period. Eventually, by the later Permian Period, this drift would lead to collision with the equatorial continent known as Euramerica, forming Pangaea.
The mountain building of the Caledonian Orogeny, a collision between Euramerica and the smaller northern continent of Siberia, continued in what would later be Great Britain, the northern Appalachians and the Nordic mountains. Rapid erosion of these mountains contributed large amounts of sediment to lowlands and shallow ocean basins. Sea levels were high with much of western North America under water. Climate of the continental interior regions was very warm during the Devonian Period and generally quite dry.
Plants, which had begun colonizing the land during the Silurian Period, continued to make evolutionary progress during the Devonian. Lycophytes, horsetails and ferns grew to large sizes and formed Earth’s first forests. By the end of the Devonian, progymnosperms such as Archaeopteris were the first successful trees. Archaeopteris could grow up to 98 feet (30 meters) tall with a trunk diameter of more than 3 feet. It had a softwood trunk similar to modern conifers that grew in sequential rings. It did not have true leaves but fern-like structures connected directly to the branches (lacking the stems of true leaves). There is evidence that they were deciduous, as the most common fossils are shed branches. Reproduction was by male and female spores that are accepted as being the precursors to seed-bearing plants. By the end of the Devonian Period, the proliferation of plants increased the oxygen content of the atmosphere considerably, which was important for development of terrestrial animals. At the same time carbon dioxide (CO2), a greenhouse gas, was depleted from earlier levels. This may have contributed to the cooling climate and the extinction event at the end of the Devonian.
Arthropod fossils are concurrent with the earliest plant fossils of the Silurian. Millipedes, centipedes and arachnids continued to diversify during the Devonian Period. The earliest known insect, Rhyniella praecusor, was a flightless hexapod with antennae and a segmented body. Fossil Rhyniella are between 412 million and 391 million years old.
Early tetrapods probably evolved from Lobe-finned fishes able to use their muscular fins to take advantage of the predator-free and food-rich environment of the new wetland ecosystems. The earliest known tetrapod is Tiktaalik rosae. Dated from the mid-Devonian, this fossil creature is considered to be the link between the lobe-finned fishes and early amphibians. Tiktaalik was probably mostly aquatic, “walking” on the bottom of shallow water estuaries. It had a fish-like pelvis, but its hind limbs were larger and stronger than those in front, suggesting it was able to propel itself outside of an aquatic environment. It had a crocodile-like head, a moveable neck, and nostrils for breathing air.
Also read: Paleozoic Era: Facts & Information